Techniques of active noise cancellation and noise compensation have been proposed to improve headphone playback quality in the presence of external interferences. Approaches of combining the noise compensation and the active noise cancellation have been proposed to benefit from both of them, for example, in WO2013144099A1, “Apparatus and Method for Improving the Perceived Quality of Sound Reproduction by Combining Active Noise Cancellation and Perceptual Noise compensation,” the contents of which are incorporated by reference herein in their entirety.
FIG. 1 is a block diagram illustrating an example of an audio processing device 100 for performing combined active noise cancellation and noise compensation in a headphone. As illustrated in FIG. 1, the audio processing device 100 includes a noise compensation unit 101, an active noise cancellation unit 102 and a combiner 103.
The noise compensation unit 101 aims to enhance the headphone playback quality by boosting an audio signal (also called content audio signal) to be played by the headphone. Specifically, the noise compensation unit 101 is configured to obtain the spectral characteristics of a noise signal captured from the external noise and selectively adjust the playback level of the audio signal by deriving one or more gains in response. In determining the gains, in spectral regions in which the background noise is not deemed as distracting, the audio signal is left largely unmodified. However, in spectral regions in which the background noise level is high enough to negatively affect the perceived quality or audibility of the audio signal, a level adjustment is made to the audio signal to improve the audio quality for the listener.
The active noise cancellation unit 102 aims to enhance the headphone playback quality by reducing the noise level arriving at the listeners' eardrum. That is, it tries to generate a signal that has the inverse phase with the residual noise, that is, external noise leaked through the headphone, and thus reduce the amount of noise arriving at the eardrum. In summary, it targets to enhance the audio quality by attenuating the level of the undesired external noise. It should be noted that the effectiveness of the noise cancellation technique is limited below (around) 1500 Hz. Cancellation of the higher frequency noise is constrained by hardware limitations.
The combiner 103 combines the audio signal adjusted by the noise compensation unit 101 and the signal generated by the active noise cancellation unit 102 and outputs a combined audio signal so as to reproduce it through a speaker 104 of the headphone. Alternatively, the combiner 103 may be incorporated in the active noise cancellation unit 102. In this situation, the active noise cancellation unit 102 may give boost to the audio signal played by the headphone to further enhance the signal to noise ratio.